Rename core.repo-finders to core.default-repo-finders

This renames a config key to make its semantics more obvious. Despite
what the commit message says, it only applies when a set of repo finders
is not specified (either on the command line or in a library API call).
This also renames the corresponding ostree_repo_get function. We can do
this since it hasn't been released yet.

Closes: #1763
Approved by: pwithnall

man/ostree.repo-config.xml

@@ -217,7 +217,7 @@ Boston, MA 02111-1307, USA.
       </varlistentry>
 
       <varlistentry>
-        <term><varname>repo-finders</varname></term>
+        <term><varname>default-repo-finders</varname></term>
         <listitem><para>Semicolon separated default list of finders (sources
         for refs) to use when pulling. This can be used to disable
         pulling from mounted filesystems, peers on the local network,